[PATCH 1/2] Update ax_cv_cxx_compile_cxx.m4

[PATCH 1/2] Update ax_cv_cxx_compile_cxx.m4

[Simon Marchi]

This file provides the AX_CXX_COMPILE_STDCXX macro.  In the context of
the following patch, I wanted to build and test GDB in c++17 mode.  The
version of the macro we have in the repo does not support detecting
c++17 compilers, but the upstream version has been updated to do so.

Since we have local modifications to the file, I had to reconcile our
modifications and the updated upstream version (which was relatively
straightforward).

gdb/ChangeLog:

	* ax_cxx_compile_stdcxx.m4: Sync with upstream.
	* configure: Re-generate.
---
 gdb/ax_cxx_compile_stdcxx.m4 | 458 +++++++++++++++++++++++++++++++++++++++++--
 gdb/configure                |  28 +--
 2 files changed, 455 insertions(+), 31 deletions(-)

diff --git a/gdb/ax_cxx_compile_stdcxx.m4 b/gdb/ax_cxx_compile_stdcxx.m4
index 444799a64678..9a9e9e713a53 100644
--- a/gdb/ax_cxx_compile_stdcxx.m4
+++ b/gdb/ax_cxx_compile_stdcxx.m4
@@ -8,7 +8,7 @@
 # - AC_SUBST CXX_DIALECT instead of changing CXX/CXXCPP.
 #
 # ===========================================================================
-#   http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
+#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -42,21 +42,22 @@
 #   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov@google.com>
 #   Copyright (c) 2015 Paul Norman <penorman@mac.com>
 #   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
+#   Copyright (c) 2016 Krzesimir Nowak <qdlacz@gmail.com>
 #
 #   Copying and distribution of this file, with or without modification, are
 #   permitted in any medium without royalty provided the copyright notice
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-#serial 4
+#serial 8
 
 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl  (serial version number 13).
 
 AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
-  m4_if([$1], [11], [],
-        [$1], [14], [],
-        [$1], [17], [m4_fatal([support for C++17 not yet implemented in AX_CXX_COMPILE_STDCXX])],
+  m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
+        [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
+        [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
   m4_if([$2], [], [],
         [$2], [ext], [],
@@ -80,7 +81,8 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
 
   m4_if([$2], [noext], [], [dnl
   if test x$ac_success = xno; then
-    for switch in -std=gnu++$1 -std=gnu++0x; do
+    for alternative in ${ax_cxx_compile_alternatives}; do
+      switch="-std=gnu++${alternative}"
       cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
       AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                      $cachevar,
@@ -103,19 +105,24 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
     dnl HP's aCC needs +std=c++11 according to:
     dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
     dnl Cray's crayCC needs "-h std=c++11"
-    for switch in -std=c++$1 -std=c++0x +std=c++$1 "-h std=c++$1"; do
-      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
-      AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
-                     $cachevar,
-        [ac_save_CXX="$CXX"
-         CXX="$CXX $switch"
-         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
-          [eval $cachevar=yes],
-          [eval $cachevar=no])
-         CXX="$ac_save_CXX"])
-      if eval test x\$$cachevar = xyes; then
-        CXX_DIALECT="$switch"
-        ac_success=yes
+    for alternative in ${ax_cxx_compile_alternatives}; do
+      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
+        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
+        AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
+                       $cachevar,
+          [ac_save_CXX="$CXX"
+           CXX="$CXX $switch"
+           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
+            [eval $cachevar=yes],
+            [eval $cachevar=no])
+           CXX="$ac_save_CXX"])
+        if eval test x\$$cachevar = xyes; then
+          CXX_DIALECT="$switch"
+          ac_success=yes
+          break
+        fi
+      done
+      if test x$ac_success = xyes; then
         break
       fi
     done
@@ -153,6 +160,11 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
 )
 
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+)
 
 dnl  Tests for new features in C++11
 
@@ -523,7 +535,7 @@ namespace cxx14
 
   }
 
-  namespace test_digit_seperators
+  namespace test_digit_separators
   {
 
     constexpr auto ten_million = 100'000'000;
@@ -565,3 +577,409 @@ namespace cxx14
 #endif  // __cplusplus >= 201402L
 
 ]])
+
+
+dnl  Tests for new features in C++17
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
+
+// If the compiler admits that it is not ready for C++17, why torture it?
+// Hopefully, this will speed up the test.
+
+#ifndef __cplusplus
+
+#error "This is not a C++ compiler"
+
+#elif __cplusplus <= 201402L
+
+#error "This is not a C++17 compiler"
+
+#else
+
+#if defined(__clang__)
+  #define REALLY_CLANG
+#else
+  #if defined(__GNUC__)
+    #define REALLY_GCC
+  #endif
+#endif
+
+#include <initializer_list>
+#include <utility>
+#include <type_traits>
+
+namespace cxx17
+{
+
+#if !defined(REALLY_CLANG)
+  namespace test_constexpr_lambdas
+  {
+
+    // TODO: test it with clang++ from git
+
+    constexpr int foo = [](){return 42;}();
+
+  }
+#endif // !defined(REALLY_CLANG)
+
+  namespace test::nested_namespace::definitions
+  {
+
+  }
+
+  namespace test_fold_expression
+  {
+
+    template<typename... Args>
+    int multiply(Args... args)
+    {
+      return (args * ... * 1);
+    }
+
+    template<typename... Args>
+    bool all(Args... args)
+    {
+      return (args && ...);
+    }
+
+  }
+
+  namespace test_extended_static_assert
+  {
+
+    static_assert (true);
+
+  }
+
+  namespace test_auto_brace_init_list
+  {
+
+    auto foo = {5};
+    auto bar {5};
+
+    static_assert(std::is_same<std::initializer_list<int>, decltype(foo)>::value);
+    static_assert(std::is_same<int, decltype(bar)>::value);
+  }
+
+  namespace test_typename_in_template_template_parameter
+  {
+
+    template<template<typename> typename X> struct D;
+
+  }
+
+  namespace test_fallthrough_nodiscard_maybe_unused_attributes
+  {
+
+    int f1()
+    {
+      return 42;
+    }
+
+    [[nodiscard]] int f2()
+    {
+      [[maybe_unused]] auto unused = f1();
+
+      switch (f1())
+      {
+      case 17:
+        f1();
+        [[fallthrough]];
+      case 42:
+        f1();
+      }
+      return f1();
+    }
+
+  }
+
+  namespace test_extended_aggregate_initialization
+  {
+
+    struct base1
+    {
+      int b1, b2 = 42;
+    };
+
+    struct base2
+    {
+      base2() {
+        b3 = 42;
+      }
+      int b3;
+    };
+
+    struct derived : base1, base2
+    {
+        int d;
+    };
+
+    derived d1 {{1, 2}, {}, 4};  // full initialization
+    derived d2 {{}, {}, 4};      // value-initialized bases
+
+  }
+
+  namespace test_general_range_based_for_loop
+  {
+
+    struct iter
+    {
+      int i;
+
+      int& operator* ()
+      {
+        return i;
+      }
+
+      const int& operator* () const
+      {
+        return i;
+      }
+
+      iter& operator++()
+      {
+        ++i;
+        return *this;
+      }
+    };
+
+    struct sentinel
+    {
+      int i;
+    };
+
+    bool operator== (const iter& i, const sentinel& s)
+    {
+      return i.i == s.i;
+    }
+
+    bool operator!= (const iter& i, const sentinel& s)
+    {
+      return !(i == s);
+    }
+
+    struct range
+    {
+      iter begin() const
+      {
+        return {0};
+      }
+
+      sentinel end() const
+      {
+        return {5};
+      }
+    };
+
+    void f()
+    {
+      range r {};
+
+      for (auto i : r)
+      {
+        [[maybe_unused]] auto v = i;
+      }
+    }
+
+  }
+
+  namespace test_lambda_capture_asterisk_this_by_value
+  {
+
+    struct t
+    {
+      int i;
+      int foo()
+      {
+        return [*this]()
+        {
+          return i;
+        }();
+      }
+    };
+
+  }
+
+  namespace test_enum_class_construction
+  {
+
+    enum class byte : unsigned char
+    {};
+
+    byte foo {42};
+
+  }
+
+  namespace test_constexpr_if
+  {
+
+    template <bool cond>
+    int f ()
+    {
+      if constexpr(cond)
+      {
+        return 13;
+      }
+      else
+      {
+        return 42;
+      }
+    }
+
+  }
+
+  namespace test_selection_statement_with_initializer
+  {
+
+    int f()
+    {
+      return 13;
+    }
+
+    int f2()
+    {
+      if (auto i = f(); i > 0)
+      {
+        return 3;
+      }
+
+      switch (auto i = f(); i + 4)
+      {
+      case 17:
+        return 2;
+
+      default:
+        return 1;
+      }
+    }
+
+  }
+
+#if !defined(REALLY_CLANG)
+  namespace test_template_argument_deduction_for_class_templates
+  {
+
+    // TODO: test it with clang++ from git
+
+    template <typename T1, typename T2>
+    struct pair
+    {
+      pair (T1 p1, T2 p2)
+        : m1 {p1},
+          m2 {p2}
+      {}
+
+      T1 m1;
+      T2 m2;
+    };
+
+    void f()
+    {
+      [[maybe_unused]] auto p = pair{13, 42u};
+    }
+
+  }
+#endif // !defined(REALLY_CLANG)
+
+  namespace test_non_type_auto_template_parameters
+  {
+
+    template <auto n>
+    struct B
+    {};
+
+    B<5> b1;
+    B<'a'> b2;
+
+  }
+
+#if !defined(REALLY_CLANG)
+  namespace test_structured_bindings
+  {
+
+    // TODO: test it with clang++ from git
+
+    int arr[2] = { 1, 2 };
+    std::pair<int, int> pr = { 1, 2 };
+
+    auto f1() -> int(&)[2]
+    {
+      return arr;
+    }
+
+    auto f2() -> std::pair<int, int>&
+    {
+      return pr;
+    }
+
+    struct S
+    {
+      int x1 : 2;
+      volatile double y1;
+    };
+
+    S f3()
+    {
+      return {};
+    }
+
+    auto [ x1, y1 ] = f1();
+    auto& [ xr1, yr1 ] = f1();
+    auto [ x2, y2 ] = f2();
+    auto& [ xr2, yr2 ] = f2();
+    const auto [ x3, y3 ] = f3();
+
+  }
+#endif // !defined(REALLY_CLANG)
+
+#if !defined(REALLY_CLANG)
+  namespace test_exception_spec_type_system
+  {
+
+    // TODO: test it with clang++ from git
+
+    struct Good {};
+    struct Bad {};
+
+    void g1() noexcept;
+    void g2();
+
+    template<typename T>
+    Bad
+    f(T*, T*);
+
+    template<typename T1, typename T2>
+    Good
+    f(T1*, T2*);
+
+    static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);
+
+  }
+#endif // !defined(REALLY_CLANG)
+
+  namespace test_inline_variables
+  {
+
+    template<class T> void f(T)
+    {}
+
+    template<class T> inline T g(T)
+    {
+      return T{};
+    }
+
+    template<> inline void f<>(int)
+    {}
+
+    template<> int g<>(int)
+    {
+      return 5;
+    }
+
+  }
+
+}  // namespace cxx17
+
+#endif  // __cplusplus <= 201402L
+
+]])
diff --git a/gdb/configure b/gdb/configure
index ab5818785333..12b626aee30c 100755
--- a/gdb/configure
+++ b/gdb/configure
@@ -4967,7 +4967,7 @@ program_transform_name=`$as_echo "$program_transform_name" | sed "$ac_script"`
 # We require a C++11 compiler.  Check if one is available, and if
 # necessary, set CXX_DIALECT to some -std=xxx switch.
 
-      ax_cxx_compile_cxx11_required=true
+  ax_cxx_compile_alternatives="11 0x"    ax_cxx_compile_cxx11_required=true
   ac_ext=cpp
 ac_cpp='$CXXCPP $CPPFLAGS'
 ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -5283,7 +5283,8 @@ $as_echo "$ax_cv_cxx_compile_cxx11" >&6; }
   fi
 
     if test x$ac_success = xno; then
-    for switch in -std=gnu++11 -std=gnu++0x; do
+    for alternative in ${ax_cxx_compile_alternatives}; do
+      switch="-std=gnu++${alternative}"
       cachevar=`$as_echo "ax_cv_cxx_compile_cxx11_$switch" | $as_tr_sh`
       { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CXX supports C++11 features with $switch" >&5
 $as_echo_n "checking whether $CXX supports C++11 features with $switch... " >&6; }
@@ -5601,16 +5602,17 @@ $as_echo "$ac_res" >&6; }
   fi
 
     if test x$ac_success = xno; then
-                for switch in -std=c++11 -std=c++0x +std=c++11 "-h std=c++11"; do
-      cachevar=`$as_echo "ax_cv_cxx_compile_cxx11_$switch" | $as_tr_sh`
-      { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CXX supports C++11 features with $switch" >&5
+                for alternative in ${ax_cxx_compile_alternatives}; do
+      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
+        cachevar=`$as_echo "ax_cv_cxx_compile_cxx11_$switch" | $as_tr_sh`
+        { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CXX supports C++11 features with $switch" >&5
 $as_echo_n "checking whether $CXX supports C++11 features with $switch... " >&6; }
 if { as_var=$cachevar; eval "test \"\${$as_var+set}\" = set"; }; then :
   $as_echo_n "(cached) " >&6
 else
   ac_save_CXX="$CXX"
-         CXX="$CXX $switch"
-         cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+           CXX="$CXX $switch"
+           cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
 
@@ -5905,14 +5907,18 @@ else
   eval $cachevar=no
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-         CXX="$ac_save_CXX"
+           CXX="$ac_save_CXX"
 fi
 eval ac_res=\$$cachevar
 	       { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_res" >&5
 $as_echo "$ac_res" >&6; }
-      if eval test x\$$cachevar = xyes; then
-        CXX_DIALECT="$switch"
-        ac_success=yes
+        if eval test x\$$cachevar = xyes; then
+          CXX_DIALECT="$switch"
+          ac_success=yes
+          break
+        fi
+      done
+      if test x$ac_success = xyes; then
         break
       fi
     done
-- 
2.16.2

[PATCH 2/2] Add gdb::string_view

[Simon Marchi]

We had a few times the need for a data structure that does essentially
what C++17's std::string_view does, which is to give an std::string-like
interface (only the read-only operations) to an arbitrary character
buffer.

I first copied the string_view file from today's gcc master
(b427286632d7) and adapted it (I don't think there should be any legal
issues since the copyright should already belong to the FSF):

- I removed things related to wstring_view, u16string_view and
  u32string_view (I don't think we need them, but we can always add them
  later).
- I removed usages of _GLIBCXX_BEGIN_NAMESPACE_VERSION and
  _GLIBCXX_END_NAMESPACE_VERSION.
- I put the code in the gdb namespace.  I had to add a few "std::" in front of
  std type usages.
- I added a constructor that builds a string_view from an std::string,
  so that we can pass strings to string_view parameters seamlessly.
  Normally, that's handled by "operator __sv_type" in the std::string
  declaration, but it only exists when building with c++17.
- When building with >= c++17, gdb::string_view is an alias of
  std::string_view.

The result is close enough to the original file that if we ever need to
update it, it should be easy enough to compare it with the new version
in a diff editor and merge the new changes in.

gdb/ChangeLog:

	* common/string_view.h: New file.
	* cli/cli-script.c (user_args) <m_args>: Change element type to
	gdb::string_view.
---
 gdb/cli/cli-script.c     |  17 +-
 gdb/common/string_view.h | 594 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 597 insertions(+), 14 deletions(-)
 create mode 100644 gdb/common/string_view.h

diff --git a/gdb/cli/cli-script.c b/gdb/cli/cli-script.c
index 366c42227c62..e705b7a8da70 100644
--- a/gdb/cli/cli-script.c
+++ b/gdb/cli/cli-script.c
@@ -32,6 +32,7 @@
 #include "extension.h"
 #include "interps.h"
 #include "compile/compile.h"
+#include "common/string_view.h"
 
 #include <vector>
 
@@ -54,18 +55,6 @@ static int command_nest_depth = 1;
 /* This is to prevent certain commands being printed twice.  */
 static int suppress_next_print_command_trace = 0;
 
-/* A non-owning slice of a string.  */
-
-struct string_view
-{
-  string_view (const char *str_, size_t len_)
-    : str (str_), len (len_)
-  {}
-
-  const char *str;
-  size_t len;
-};
-
 /* Structure for arguments to user defined functions.  */
 
 class user_args
@@ -91,7 +80,7 @@ private:
   std::string m_command_line;
 
   /* The arguments.  Each element points inside M_COMMAND_LINE.  */
-  std::vector<string_view> m_args;
+  std::vector<gdb::string_view> m_args;
 };
 
 /* The stack of arguments passed to user defined functions.  We need a
@@ -827,7 +816,7 @@ user_args::insert_args (const char *line) const
 	    error (_("Missing argument %ld in user function."), i);
 	  else
 	    {
-	      new_line.append (m_args[i].str, m_args[i].len);
+	      new_line.append (m_args[i].data (), m_args[i].length ());
 	      line = tmp;
 	    }
 	}
diff --git a/gdb/common/string_view.h b/gdb/common/string_view.h
new file mode 100644
index 000000000000..710049dbf0eb
--- /dev/null
+++ b/gdb/common/string_view.h
@@ -0,0 +1,594 @@
+// Components for manipulating non-owning sequences of characters -*- C++ -*-
+
+// Note: This file has been stolen from the gcc repo
+// (libstdc++-v3/include/std/string_view) and has local modifications.
+
+// Copyright (C) 2013-2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+
+//
+// N3762 basic_string_view library
+//
+
+#ifndef STRING_VIEW_H
+#define STRING_VIEW_H 1
+
+#if __cplusplus >= 201703L
+
+#include <string_view>
+
+namespace gdb {
+  using string_view = std::string_view;
+} /* namespace gdb */
+
+#else /* __cplusplus >= 201703L */
+
+#include <limits>
+#include <iosfwd>
+#include <bits/char_traits.h>
+#include <bits/functional_hash.h>
+#include <bits/range_access.h>
+
+namespace gdb {
+  /**
+   *  @class basic_string_view <string_view>
+   *  @brief  A non-owning reference to a string.
+   *
+   *  @ingroup strings
+   *  @ingroup sequences
+   *
+   *  @tparam _CharT  Type of character
+   *  @tparam _Traits  Traits for character type, defaults to
+   *                   char_traits<_CharT>.
+   *
+   *  A basic_string_view looks like this:
+   *
+   *  @code
+   *    _CharT*    _M_str
+   *    size_t     _M_len
+   *  @endcode
+   */
+  template<typename _CharT, typename _Traits = std::char_traits<_CharT>>
+    class basic_string_view
+    {
+    public:
+
+      // types
+      using traits_type = _Traits;
+      using value_type = _CharT;
+      using pointer = const _CharT*;
+      using const_pointer = const _CharT*;
+      using reference = const _CharT&;
+      using const_reference = const _CharT&;
+      using const_iterator = const _CharT*;
+      using iterator = const_iterator;
+      using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+      using reverse_iterator = const_reverse_iterator;
+      using size_type = size_t;
+      using difference_type = ptrdiff_t;
+      static constexpr size_type npos = size_type(-1);
+
+      // [string.view.cons], construct/copy
+
+      constexpr
+      basic_string_view() noexcept
+      : _M_len{0}, _M_str{nullptr}
+      { }
+
+      constexpr basic_string_view(const basic_string_view&) noexcept = default;
+
+      constexpr basic_string_view(const _CharT* __str) noexcept
+      : _M_len{__str == nullptr ? 0 : traits_type::length(__str)},
+	_M_str{__str}
+      { }
+
+      constexpr
+      basic_string_view(const _CharT* __str, size_type __len) noexcept
+      : _M_len{__len}, _M_str{__str}
+      { }
+
+      constexpr
+      basic_string_view(const std::string &__str) noexcept
+      : _M_len{__str.length()}, _M_str{__str.c_str()}
+      { }
+
+      constexpr basic_string_view&
+      operator=(const basic_string_view&) noexcept = default;
+
+      // [string.view.iterators], iterators
+
+      constexpr const_iterator
+      begin() const noexcept
+      { return this->_M_str; }
+
+      constexpr const_iterator
+      end() const noexcept
+      { return this->_M_str + this->_M_len; }
+
+      constexpr const_iterator
+      cbegin() const noexcept
+      { return this->_M_str; }
+
+      constexpr const_iterator
+      cend() const noexcept
+      { return this->_M_str + this->_M_len; }
+
+      constexpr const_reverse_iterator
+      rbegin() const noexcept
+      { return const_reverse_iterator(this->end()); }
+
+      constexpr const_reverse_iterator
+      rend() const noexcept
+      { return const_reverse_iterator(this->begin()); }
+
+      constexpr const_reverse_iterator
+      crbegin() const noexcept
+      { return const_reverse_iterator(this->end()); }
+
+      constexpr const_reverse_iterator
+      crend() const noexcept
+      { return const_reverse_iterator(this->begin()); }
+
+      // [string.view.capacity], capacity
+
+      constexpr size_type
+      size() const noexcept
+      { return this->_M_len; }
+
+      constexpr size_type
+      length() const noexcept
+      { return _M_len; }
+
+      constexpr size_type
+      max_size() const noexcept
+      {
+	return (npos - sizeof(size_type) - sizeof(void*))
+		/ sizeof(value_type) / 4;
+      }
+
+      constexpr bool
+      empty() const noexcept
+      { return this->_M_len == 0; }
+
+      // [string.view.access], element access
+
+      constexpr const _CharT&
+      operator[](size_type __pos) const noexcept
+      {
+	// TODO: Assert to restore in a way compatible with the constexpr.
+	// __glibcxx_assert(__pos < this->_M_len);
+	return *(this->_M_str + __pos);
+      }
+
+      constexpr const _CharT&
+      at(size_type __pos) const
+      {
+	if (__pos >= _M_len)
+	  __throw_out_of_range_fmt(__N("basic_string_view::at: __pos "
+				       "(which is %zu) >= this->size() "
+				       "(which is %zu)"), __pos, this->size());
+	return *(this->_M_str + __pos);
+      }
+
+      constexpr const _CharT&
+      front() const noexcept
+      {
+	// TODO: Assert to restore in a way compatible with the constexpr.
+	// __glibcxx_assert(this->_M_len > 0);
+	return *this->_M_str;
+      }
+
+      constexpr const _CharT&
+      back() const noexcept
+      {
+	// TODO: Assert to restore in a way compatible with the constexpr.
+	// __glibcxx_assert(this->_M_len > 0);
+	return *(this->_M_str + this->_M_len - 1);
+      }
+
+      constexpr const _CharT*
+      data() const noexcept
+      { return this->_M_str; }
+
+      // [string.view.modifiers], modifiers:
+
+      constexpr void
+      remove_prefix(size_type __n) noexcept
+      {
+	__glibcxx_assert(this->_M_len >= __n);
+	this->_M_str += __n;
+	this->_M_len -= __n;
+      }
+
+      constexpr void
+      remove_suffix(size_type __n) noexcept
+      { this->_M_len -= __n; }
+
+      constexpr void
+      swap(basic_string_view& __sv) noexcept
+      {
+	auto __tmp = *this;
+	*this = __sv;
+	__sv = __tmp;
+      }
+
+
+      // [string.view.ops], string operations:
+
+      size_type
+      copy(_CharT* __str, size_type __n, size_type __pos = 0) const
+      {
+	__glibcxx_requires_string_len(__str, __n);
+	__pos = _M_check(__pos, "basic_string_view::copy");
+	const size_type __rlen = std::min(__n, _M_len - __pos);
+	for (auto __begin = this->_M_str + __pos,
+	     __end = __begin + __rlen; __begin != __end;)
+	  *__str++ = *__begin++;
+	return __rlen;
+      }
+
+
+      // [string.view.ops], string operations:
+
+      constexpr basic_string_view
+      substr(size_type __pos, size_type __n = npos) const noexcept(false)
+      {
+	__pos = _M_check(__pos, "basic_string_view::substr");
+	const size_type __rlen = std::min(__n, _M_len - __pos);
+	return basic_string_view{_M_str + __pos, __rlen};
+      }
+
+      constexpr int
+      compare(basic_string_view __str) const noexcept
+      {
+	const size_type __rlen = std::min(this->_M_len, __str._M_len);
+	int __ret = traits_type::compare(this->_M_str, __str._M_str, __rlen);
+	if (__ret == 0)
+	  __ret = _S_compare(this->_M_len, __str._M_len);
+	return __ret;
+      }
+
+      constexpr int
+      compare(size_type __pos1, size_type __n1, basic_string_view __str) const
+      { return this->substr(__pos1, __n1).compare(__str); }
+
+      constexpr int
+      compare(size_type __pos1, size_type __n1,
+	      basic_string_view __str, size_type __pos2, size_type __n2) const
+      {
+	return this->substr(__pos1, __n1).compare(__str.substr(__pos2, __n2));
+      }
+
+      constexpr int
+      compare(const _CharT* __str) const noexcept
+      { return this->compare(basic_string_view{__str}); }
+
+      constexpr int
+      compare(size_type __pos1, size_type __n1, const _CharT* __str) const
+      { return this->substr(__pos1, __n1).compare(basic_string_view{__str}); }
+
+      constexpr int
+      compare(size_type __pos1, size_type __n1,
+	      const _CharT* __str, size_type __n2) const noexcept(false)
+      {
+	return this->substr(__pos1, __n1)
+		   .compare(basic_string_view(__str, __n2));
+      }
+
+      constexpr size_type
+      find(basic_string_view __str, size_type __pos = 0) const noexcept
+      { return this->find(__str._M_str, __pos, __str._M_len); }
+
+      constexpr size_type
+      find(_CharT __c, size_type __pos = 0) const noexcept;
+
+      constexpr size_type
+      find(const _CharT* __str, size_type __pos, size_type __n) const noexcept;
+
+      constexpr size_type
+      find(const _CharT* __str, size_type __pos = 0) const noexcept
+      { return this->find(__str, __pos, traits_type::length(__str)); }
+
+      constexpr size_type
+      rfind(basic_string_view __str, size_type __pos = npos) const noexcept
+      { return this->rfind(__str._M_str, __pos, __str._M_len); }
+
+      constexpr size_type
+      rfind(_CharT __c, size_type __pos = npos) const noexcept;
+
+      constexpr size_type
+      rfind(const _CharT* __str, size_type __pos, size_type __n) const noexcept;
+
+      constexpr size_type
+      rfind(const _CharT* __str, size_type __pos = npos) const noexcept
+      { return this->rfind(__str, __pos, traits_type::length(__str)); }
+
+      constexpr size_type
+      find_first_of(basic_string_view __str, size_type __pos = 0) const noexcept
+      { return this->find_first_of(__str._M_str, __pos, __str._M_len); }
+
+      constexpr size_type
+      find_first_of(_CharT __c, size_type __pos = 0) const noexcept
+      { return this->find(__c, __pos); }
+
+      constexpr size_type
+      find_first_of(const _CharT* __str, size_type __pos, size_type __n) const noexcept;
+
+      constexpr size_type
+      find_first_of(const _CharT* __str, size_type __pos = 0) const noexcept
+      { return this->find_first_of(__str, __pos, traits_type::length(__str)); }
+
+      constexpr size_type
+      find_last_of(basic_string_view __str,
+		   size_type __pos = npos) const noexcept
+      { return this->find_last_of(__str._M_str, __pos, __str._M_len); }
+
+      constexpr size_type
+      find_last_of(_CharT __c, size_type __pos=npos) const noexcept
+      { return this->rfind(__c, __pos); }
+
+      constexpr size_type
+      find_last_of(const _CharT* __str, size_type __pos,
+		   size_type __n) const noexcept;
+
+      constexpr size_type
+      find_last_of(const _CharT* __str, size_type __pos = npos) const noexcept
+      { return this->find_last_of(__str, __pos, traits_type::length(__str)); }
+
+      constexpr size_type
+      find_first_not_of(basic_string_view __str,
+			size_type __pos = 0) const noexcept
+      { return this->find_first_not_of(__str._M_str, __pos, __str._M_len); }
+
+      constexpr size_type
+      find_first_not_of(_CharT __c, size_type __pos = 0) const noexcept;
+
+      constexpr size_type
+      find_first_not_of(const _CharT* __str,
+			size_type __pos, size_type __n) const noexcept;
+
+      constexpr size_type
+      find_first_not_of(const _CharT* __str, size_type __pos = 0) const noexcept
+      {
+	return this->find_first_not_of(__str, __pos,
+				       traits_type::length(__str));
+      }
+
+      constexpr size_type
+      find_last_not_of(basic_string_view __str,
+		       size_type __pos = npos) const noexcept
+      { return this->find_last_not_of(__str._M_str, __pos, __str._M_len); }
+
+      constexpr size_type
+      find_last_not_of(_CharT __c, size_type __pos = npos) const noexcept;
+
+      constexpr size_type
+      find_last_not_of(const _CharT* __str,
+		       size_type __pos, size_type __n) const noexcept;
+
+      constexpr size_type
+      find_last_not_of(const _CharT* __str,
+		       size_type __pos = npos) const noexcept
+      {
+	return this->find_last_not_of(__str, __pos,
+				      traits_type::length(__str));
+      }
+
+      constexpr size_type
+      _M_check(size_type __pos, const char* __s) const noexcept(false)
+      {
+	if (__pos > this->size())
+	  __throw_out_of_range_fmt(__N("%s: __pos (which is %zu) > "
+				       "this->size() (which is %zu)"),
+				   __s, __pos, this->size());
+	return __pos;
+      }
+
+      // NB: _M_limit doesn't check for a bad __pos value.
+      constexpr size_type
+      _M_limit(size_type __pos, size_type __off) const noexcept
+      {
+	const bool __testoff =  __off < this->size() - __pos;
+	return __testoff ? __off : this->size() - __pos;
+      }
+      
+    private:
+
+      static constexpr int
+      _S_compare(size_type __n1, size_type __n2) noexcept
+      {
+	const difference_type __diff = __n1 - __n2;
+	if (__diff > std::numeric_limits<int>::max())
+	  return std::numeric_limits<int>::max();
+	if (__diff < std::numeric_limits<int>::min())
+	  return std::numeric_limits<int>::min();
+	return static_cast<int>(__diff);
+      }
+
+      size_t	    _M_len;
+      const _CharT* _M_str;
+    };
+
+  // [string.view.comparison], non-member basic_string_view comparison function
+
+  namespace __detail
+  {
+    // Identity transform to create a non-deduced context, so that only one
+    // argument participates in template argument deduction and the other
+    // argument gets implicitly converted to the deduced type. See n3766.html.
+    template<typename _Tp>
+      using __idt = std::common_type_t<_Tp>;
+  }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator==(basic_string_view<_CharT, _Traits> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.size() == __y.size() && __x.compare(__y) == 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator==(basic_string_view<_CharT, _Traits> __x,
+               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
+    { return __x.size() == __y.size() && __x.compare(__y) == 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator==(__detail::__idt<basic_string_view<_CharT, _Traits>> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.size() == __y.size() && __x.compare(__y) == 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator!=(basic_string_view<_CharT, _Traits> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return !(__x == __y); }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator!=(basic_string_view<_CharT, _Traits> __x,
+               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
+    { return !(__x == __y); }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator!=(__detail::__idt<basic_string_view<_CharT, _Traits>> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return !(__x == __y); }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator< (basic_string_view<_CharT, _Traits> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) < 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator< (basic_string_view<_CharT, _Traits> __x,
+               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
+    { return __x.compare(__y) < 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator< (__detail::__idt<basic_string_view<_CharT, _Traits>> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) < 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator> (basic_string_view<_CharT, _Traits> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) > 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator> (basic_string_view<_CharT, _Traits> __x,
+               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
+    { return __x.compare(__y) > 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator> (__detail::__idt<basic_string_view<_CharT, _Traits>> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) > 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator<=(basic_string_view<_CharT, _Traits> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) <= 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator<=(basic_string_view<_CharT, _Traits> __x,
+               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
+    { return __x.compare(__y) <= 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator<=(__detail::__idt<basic_string_view<_CharT, _Traits>> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) <= 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator>=(basic_string_view<_CharT, _Traits> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) >= 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator>=(basic_string_view<_CharT, _Traits> __x,
+               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
+    { return __x.compare(__y) >= 0; }
+
+  template<typename _CharT, typename _Traits>
+    constexpr bool
+    operator>=(__detail::__idt<basic_string_view<_CharT, _Traits>> __x,
+               basic_string_view<_CharT, _Traits> __y) noexcept
+    { return __x.compare(__y) >= 0; }
+
+  // [string.view.io], Inserters and extractors
+  template<typename _CharT, typename _Traits>
+    inline std::basic_ostream<_CharT, _Traits>&
+    operator<<(std::basic_ostream<_CharT, _Traits>& __os,
+	       basic_string_view<_CharT,_Traits> __str)
+    { return __ostream_insert(__os, __str.data(), __str.size()); }
+
+
+  // basic_string_view typedef names
+
+  using string_view = basic_string_view<char>;
+
+  // [string.view.hash], hash support:
+
+  template<typename _Tp>
+    struct hash;
+
+  template<>
+    struct hash<string_view>
+    : public std::__hash_base<size_t, string_view>
+    {
+      size_t
+      operator()(const string_view& __str) const noexcept
+      { return std::_Hash_impl::hash(__str.data(), __str.length()); }
+    };
+
+  inline namespace literals
+  {
+  inline namespace string_view_literals
+  {
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wliteral-suffix"
+    inline constexpr basic_string_view<char>
+    operator""sv(const char* __str, size_t __len) noexcept
+    { return basic_string_view<char>{__str, __len}; }
+#pragma GCC diagnostic pop
+  } // namespace string_literals
+  } // namespace literals
+
+} /* namespace gdb */
+
+#endif /* __cplusplus >= 201703L */
+
+#endif /* STRING_VIEW_H */
-- 
2.16.2

Re: [PATCH 2/2] Add gdb::string_view

[Simon Marchi]

On 2018-03-17 19:49, Simon Marchi wrote:
> We had a few times the need for a data structure that does essentially
> what C++17's std::string_view does, which is to give an std::string-like
> interface (only the read-only operations) to an arbitrary character
> buffer.
>
> I first copied the string_view file from today's gcc master
> (b427286632d7) and adapted it (I don't think there should be any legal
> issues since the copyright should already belong to the FSF):
>
> - I removed things related to wstring_view, u16string_view and
>   u32string_view (I don't think we need them, but we can always add them
>   later).
> - I removed usages of _GLIBCXX_BEGIN_NAMESPACE_VERSION and
>   _GLIBCXX_END_NAMESPACE_VERSION.
> - I put the code in the gdb namespace.  I had to add a few "std::" in front of
>   std type usages.
> - I added a constructor that builds a string_view from an std::string,
>   so that we can pass strings to string_view parameters seamlessly.
>   Normally, that's handled by "operator __sv_type" in the std::string
>   declaration, but it only exists when building with c++17.
> - When building with >= c++17, gdb::string_view is an alias of
>   std::string_view.
>
> The result is close enough to the original file that if we ever need to
> update it, it should be easy enough to compare it with the new version
> in a diff editor and merge the new changes in.

Hmm, when building with older g++ (such as the aarch64 builders on the buildbot,
which have g++ 4.8), it trips on:

  using __idt = std::common_type_t<_Tp>;

It looks like that release of g++ didn't have std::common_type_t.  I guess it
would be possible to avoid using it, and change these:

    operator==(basic_string_view<_CharT, _Traits> __x,
               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept

for

    operator==(basic_string_view<_CharT, _Traits> __x,
               basic_string_view<_CharT, _Traits> __y) noexcept

but I am not aware of what consequences it would have.

Simon

Re: [PATCH 2/2] Add gdb::string_view

[Pedro Alves]

On 03/18/2018 12:10 AM, Simon Marchi wrote:
> On 2018-03-17 19:49, Simon Marchi wrote:
>> We had a few times the need for a data structure that does essentially
>> what C++17's std::string_view does, which is to give an std::string-like
>> interface (only the read-only operations) to an arbitrary character
>> buffer.

Great, I've been wanting to do something like this for a while.

>>
>> I first copied the string_view file from today's gcc master
>> (b427286632d7) and adapted it (I don't think there should be any legal
>> issues since the copyright should already belong to the FSF):
>>
>> - I removed things related to wstring_view, u16string_view and
>>   u32string_view (I don't think we need them, but we can always add them
>>   later).
>> - I removed usages of _GLIBCXX_BEGIN_NAMESPACE_VERSION and
>>   _GLIBCXX_END_NAMESPACE_VERSION.
>> - I put the code in the gdb namespace.  I had to add a few "std::" in front of
>>   std type usages.
>> - I added a constructor that builds a string_view from an std::string,
>>   so that we can pass strings to string_view parameters seamlessly.
>>   Normally, that's handled by "operator __sv_type" in the std::string
>>   declaration, but it only exists when building with c++17.
>> - When building with >= c++17, gdb::string_view is an alias of
>>   std::string_view.
>>
>> The result is close enough to the original file that if we ever need to
>> update it, it should be easy enough to compare it with the new version
>> in a diff editor and merge the new changes in.
> 
> Hmm, when building with older g++ (such as the aarch64 builders on the buildbot,
> which have g++ 4.8), it trips on:
> 
>   using __idt = std::common_type_t<_Tp>;
> 
> It looks like that release of g++ didn't have std::common_type_t.  I guess it
> would be possible to avoid using it, and change these:

It has std::common_type though.  C++14 std::foo_t types are usually just a
helper/convenience alias template, like:

  /// Alias template for common_type
  template<typename... _Tp>
    using common_type_t = typename common_type<_Tp...>::type;

So it sounds like we can just use the C++11 / ::type form directly, or
add gdb::common_type_t somewhere, like common/traits.h or to our
copy of string_view.

> 
>     operator==(basic_string_view<_CharT, _Traits> __x,
>                __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept
> 
> for
> 
>     operator==(basic_string_view<_CharT, _Traits> __x,
>                basic_string_view<_CharT, _Traits> __y) noexcept
> 
> but I am not aware of what consequences it would have.

Would it be possible to import some of the libstdc++'s relevant
testscases into our unit tests framework, like was done for
gdb::optional [1]?

[1] https://sourceware.org/ml/gdb-patches/2017-04/msg00239.html

Thanks,
Pedro Alves

Re: [PATCH 2/2] Add gdb::string_view

[Simon Marchi]

On 2018-03-19 08:21, Pedro Alves wrote:
>> Hmm, when building with older g++ (such as the aarch64 builders on the 
>> buildbot,
>> which have g++ 4.8), it trips on:
>> 
>>   using __idt = std::common_type_t<_Tp>;
>> 
>> It looks like that release of g++ didn't have std::common_type_t.  I 
>> guess it
>> would be possible to avoid using it, and change these:
> 
> It has std::common_type though.  C++14 std::foo_t types are usually 
> just a
> helper/convenience alias template, like:
> 
>   /// Alias template for common_type
>   template<typename... _Tp>
>     using common_type_t = typename common_type<_Tp...>::type;
> 
> So it sounds like we can just use the C++11 / ::type form directly, or
> add gdb::common_type_t somewhere, like common/traits.h or to our
> copy of string_view.

Ok thanks I think I got it to work now.

> 
>> 
>>     operator==(basic_string_view<_CharT, _Traits> __x,
>>                __detail::__idt<basic_string_view<_CharT, _Traits>> 
>> __y) noexcept
>> 
>> for
>> 
>>     operator==(basic_string_view<_CharT, _Traits> __x,
>>                basic_string_view<_CharT, _Traits> __y) noexcept
>> 
>> but I am not aware of what consequences it would have.
> 
> Would it be possible to import some of the libstdc++'s relevant
> testscases into our unit tests framework, like was done for
> gdb::optional [1]?
> 
> [1] https://sourceware.org/ml/gdb-patches/2017-04/msg00239.html

Good idea, I did it and it caught a few important compilation issues!

I just need to clean up the patches and I'll send a v2.

Thanks,

Simon